U.S. Pat. No. 4,595,496 to William W. Carson (Carson) gives a good summary of liquid chromatography (LC), which is one important application of the present invention. The entire disclosure of said patent is hereby incorporated by reference into this application as though set in full herein.
In LC, a liquid sample is placed (injected) into a flowing stream of a liquid carrier (mobile phase), that flows through a separation device such as a packed column or membrane (stationary phase). While passing through the separation device, the various components of the sample are adsorbed and desorbed from the stationary phase at different rates thereby separating the components into different volumes (and times) eluting from the separation device. The various components then flow through a detector (or several detectors) that respond to the components, thereby providing qualitative and quantitative data about the components.
LC systems, to separate more effectively, often use fixed composition mixtures of fluids as the mobile phase (isocratic mode), and, in addition, often change the mixture composition over a total volume (or time) delivered (gradient mode) to achieve a more effective separation of the sample components.
LC systems, at least, provide for one or several fluids as mobile phases, valves to select and inject each of the fluids, a pump to deliver (delivery pump) the fluid(s) at a given flow rate, and a column of packed particles or a membrane stationary phase to separate the sample components. Often a means to detect the separated sample components is also included. In addition mixers, accumulators and other flow elements are used in specific applications.
In high pressure applications, LC systems commonly use high cost, gear driven piston pumps with high pressure sliding seals at the interface between the drive member (piston) and the housing containing the drive (cylinder). While the seals may either be fixed in position on the walls of the cylinder or disposed on the piston itself, hereinafter, "sliding seals" refers generically to those seals which separate the driven LC fluids from the outside environment, e.g., the air, regardless of how configured. Typically elements of the system are alternately wetted with the LC fluids and then with the outside environment as they pass over the seals. There will be some small amount of leakage past these seals, and if the leakage produces a residue, for example a salt from a dried buffer solution, that residue can cause wear of the seal which leads to a system failure. This occurrence is a major limitation and one of the primary failure modes or maintenance issues of such pumps.
Another limitation of LC piston pumping systems is that the alternate exposure to the fluid path and then to the outside environment provides an opportunity for extraneous substances to enter the fluid path and the LC system thus adulterating the analysis. In this manner LC system applications using sterile fluids may become non-sterile. This is of particular interest when biologically active compounds are being separated. Them is a need in such systems to eliminate this source of contamination.
Piston pumps with either single or multiple pistons also produce flow fluctuations which correspond to the reciprocating cycle of the piston(s). These fluctuations result from variations in piston velocity through the stroke and from the check valves typically used to switch the source of flow from one piston to another.
For low pressure applications, LC fluids are driven in a variety of ways, including simple gravity feed and the use of external gas pressure supplied to the fluid reservoirs. Another common low pressure drive involves the use of peristaltic pumps in which one or more rollers impinge on a tube containing the LC fluid thereby propelling the fluid through the system. Such pumps also produce flow fluctuations corresponding to the roller impingement on the fluid delivery tube. In addition, the tubes are elastic, can lose their shape or even wear out, further producing inaccurate delivery of called-for flow. Thus there is a need in these applications for longer lived mechanisms that deliver more constant fluid flows. There is also a continuing need to reduce or eliminate sliding seal leakage and to produce low cost pumps with reduced fluctuation effects.
In fluid pumping systems, especially high pressure LC systems, fluid metering is important. Such high pressure LC systems using commercially available columns generally require precise flow rates. If the flow rate varies due to hydrostatic loading, e.g. as a column fluid flow resistance changes, or as other detectors are used, the separation may be affected. Flow metering herein is defined as providing precise and repeatable flow rates. Accurate flow rates are not easily achieved for many reasons, e.g., compressibility of the fluids, non-linear mixing of the fluids, temperature dependencies. Piston pumps provide precise flow rates based on the precise machining of piston/cylinders and controlled actuation of the pumps. In order to repeat and/or verify analyses, the volume (or time) in which a sample component elutes from a column must be reproducible, so the flow rate must be reproducible. The fundamental basis of separation will determine the degree of flow accuracy which is required for reproducibility.
Both high and low pressure LC pumps generally have fluctuations in delivered flow rate, corresponding to the fundamental and harmonic frequencies of the pump driving mechanism, with periods ranging from tens of milliseconds to many seconds. This is particularly troublesome in gradient systems as flow fluctuations can result in composition fluctuations. In a gradient application, the gradient may be formed by proportioning valves on the intake side of the pump, each valve leading to a different fluid component or mixture. This is commonly referred to as low pressure gradient formation. In one configuration the proportioning valves, operating as taught by Carson, provide the desired gradient fluid compositions while minimizing the effects of flow fluctuations in the pump. In such a gradient formation system, some form of mixer is required in order to filter out residual high frequency composition fluctuations. Under-filtering (mixing) allows residual fluctuations to remain in the system. Over-filtering limits the speed of composition response of the system and complex mixers add cost.
There is a need for smoother, quicker responding gradient pumping systems, and it is advantageous to eliminate extra flow elements, like separate mixers, wherever possible.
In addition, there have been advances in membrane separation media for LC applications. U.S. Pat. No. 4,895,806, to Minh Son Le and James Alan Sanderson describes such an advancement, and the entire disclosure of said patent is hereby incorporated by reference into this application as though set in full herein. Compared to many LC separation devices, such membrane devices operate at high flow rates; lower pressures, and with separations which depend more upon fluid compositions than highly precise flow rates. These devices have also been shown to be of particular utility in the separation of biologically active compounds.
There is an opportunity for a pumping system where the pump matches the needs of such membrane separation devices. These needs include smooth precise control, rapid composition response and the elimination of composition fluctuations.
Gear pumps have been used in LC systems as a priming and/or pre-pump working with the primary system pump. In such uses, the gear pump is not the primary flow source, instead it provides an assist to maintain priming of the primary delivery pump. A typical gear pump is shown in U.S. Pat. No. 4,111,614 to Thomas B. Martin et al. The pumping characteristics and construction details of this type of pump are well known in the art. The construction of a gear pump has a leakage path from the output back to the input such that a positive pressure differential between the output and the input will force fluid to flow in opposition to the primary flow of the pump. Thus, for a given pump rotation rate, if there is a change in hydraulic resistance of the load, the resulting flow through the load will change and the difference will flow through the leakage path. If this occurs, the timing of the chromatographic separation may change necessitating continued experimentation. Commercially available gear pumps are typically limited to pressures of a few hundred pounds per square inch. Many chromatography systems require higher pressures and more easily controlled flow rates from primary delivery pumps. In addition, the construction of these lower cost gears pumps leads to performance differences from pump to pump, especially the hydraulic resistance of the output to input leakage path, which make it difficult to control the flow in a load or separation device. These preceding limitations have likely contributed to the non-use of a gear pump as the primary LC delivery pump.
However, gear pumps have advantages such as being low cost and capable of high flow rates; in addition, they may be magnetically coupled with no sliding seals thus overcoming some of the above limitations of piston pumps.
It is an object of this invention to provide a fluid gradient and fluid composition mixing system with a small delay volume.
It is another object of this invention to provide a pumping system with reduced flow fluctuation, smoother and faster gradient response. It is another object of this invention to provide such a system at a low cost.
It is another object of this invention to provide an LC system appropriate to the specific requirements of membrane based separation devices.
It is yet another object of this invention to provide pump metering systems useful at both high and low system pressures.
It is a further object of this invention to provide a pumping system which eliminates cross contamination between the pumped fluid and the external environment.
It is another object of this invention to provide an LC system with improved pump reliability.